For power generation, fuel cells utilize the energy released during the reaction in which hydrogen and oxygen combine. Thus, in view of energy conservation and environmental protection measures, it is a next generation power generating system the practical and widespread use of which is desired. There are a variety of types of fuel cells, including solid electrolyte fuel cells, molten carbonate fuel cells, phosphoric acid fuel cells, and polymer electrolyte fuel cells.
Among these, polymer electrolyte fuel cells have a higher power density and can be made more compact. Also, they operate at low temperatures and provide ease in starting and stopping as compared to other types of fuel cells. Because of this, polymer electrolyte fuel cells in particular have been given much attention in recent years as they are expected to be used in electric vehicles and home use small cogeneration equipment.
FIG. 1 is a diagram illustrating a configuration of a polymer electrolyte fuel cell (hereinafter also referred to simply as a “fuel cell”) with FIG. 1(a) being an exploded view of a unit cell that constitutes the fuel cell and FIG. (b) being an overall perspective view of the fuel cell composed of an assembly of multiple unit cells.
As shown in FIG. 1, the fuel cell 1 is a stack of unit cells. In a unit cell, as shown in FIG. 1(a), what is called an anode-side gas diffusion layer 3 or a fuel electrode 3 (hereinafter referred to simply as an “anode”) is disposed on one side of a polymer electrolyte membrane 2. On the other side of the polymer electrolyte membrane 2 is disposed what is called a cathode-side gas diffusion layer 4 or an oxidizing electrode 4 (hereinafter referred to simply as a “cathode”). The unit cell has a structure in which: the anode 3 is disposed on one side of the polymer electrolyte membrane 2 and the cathode 4 is disposed on the other side thereof; and separators (bipolar plates) 5a, 5b are disposed on the one and the other sides, respectively.
Examples of fuel cells include a water-cooled fuel cell in which a water separator having a cooling water channel is interposed between unit cells or between sets of two or more unit cells. Such a water-cooled fuel cell is also within the scope of the present invention.
As the polymer electrolyte membrane 2 (hereinafter simply referred to as “electrolyte membrane”), a fluorinated proton conducting membrane having hydrogen ion (proton) exchange groups is used. The anode 3 and cathode 4 may be provided with a catalyst layer that includes a particulate platinum catalyst, graphite powder, and optionally a fluorocarbon resin with hydrogen ion (proton) exchange groups. In this case, the reaction is promoted by contact of a fuel gas or an oxidizing gas with the catalyst layer.
A fuel gas A (hydrogen or a hydrogen containing gas) is fed through a channel 6a formed in the separator 5a to supply hydrogen to the fuel electrode 3. An oxidizing gas B such as air is fed through a channel 6b formed in the separator 5b to supply oxygen. The supply of these gases causes an electrochemical reaction to generate direct current power.
The following are major functions required of a separator of a polymer electrolyte fuel cell.
(1) a function as a “channel” for uniformly supplying a fuel gas and an oxidizing gas to the electrode surfaces;
(2) a function as a “channel” for efficiently removing water produced at the cathode side from the fuel cell system together with carrier gases such as air and oxygen after the reaction.
(3) a function of providing a path for electricity by contacting with the electrodes (anode 3 and cathode 4) and serving as an electrical “connector” between unit cells;
(4) a function as an “isolating wall” between adjacent unit cells for isolating an anode chamber of one unit cell from a cathode chamber of an adjacent unit cell; and
(5) in a water-cooled fuel cell, a function as an “isolating wall” for isolating a cooling water channel from an adjacent unit cell.
Separators for use in a polymer electrolyte fuel cell (hereinafter simply referred to as “separators”) are required to provide the above-described functions. As a base material to produce such separators, either a metal-based material or a carbon-based material is generally used.
Metal materials such as titanium have advantages of, e.g., exhibiting good workability typical of metals and thus allowing production of thinner separators, which results in production of lighter-weight separators. However, they are disadvantageous in that oxidation on the metal surface may cause a decrease in electrical conductivity. Thus, separators made from metal materials (hereinafter simply referred to as “metallic separators”) pose a problem of a possible increase in contact resistance by contact with the gas diffusion layer.
On the other hand, carbon materials have the advantage of providing light-weight separators, whereas they have disadvantages of, e.g., having gas permeability and exhibiting low mechanical strength.
With regard to metallic separators, particularly separators made of a titanium material (hereinafter simply referred to as a “titanium separator”), there are various conventional proposals as disclosed in Patent Literatures 1 to 5 listed below.
Patent Literature 1 proposes a titanium separator having a noble metal thin film, mainly of gold, formed on its surface, e.g., by plating after removal of a passivation film from the surface of the separator that is to be in contact with an electrode in order to improve corrosion resistance (resistance to oxidation). However, using large amounts of noble metal, particularly gold, in fuel cells for mobile systems such as automobiles or in stationary fuel cells is disadvantageous in view of economies and limited resources. Therefore the titanium separator proposed in Patent Literature 1 has not seen widespread use.
Patent Literature 2 proposes a solution to the problem of corrosion resistance (resistance to oxidation) of a titanium separator without the use of noble metals, particularly gold. Patent Literature 2 proposes a titanium separator having on its surface a conductive interface layer containing carbon formed by vapor deposition. However, vapor deposition is a process that requires special equipment, which leads to increased equipment costs and many hours of operation. This results in a decrease in productivity and thus causes a problem. Because of this, the titanium separator proposed in Patent Literature 2 is currently not being utilized actively.
Patent Literature 3 proposes a method for reducing the increase in contact resistance that may occur due to oxidation on the metal surface, the method including using a titanium separator having on its surface a metallic film containing dispersed electrically conductive ceramics. This material having a ceramic-containing metallic film is disadvantageous in that: in stamping a sheet blank into a separator shape, the dispersed ceramics hinder the forming process, and sometimes cracking may occur or a through-hole may be formed in the separator during the processing. In addition, since ceramic materials may cause wear of a press mold, it may become necessary to replace the press mold with one made of an expensive material such as cemented carbide. For these reasons, the titanium separator proposed in Patent Literature 3 has not been put into practical use.
Patent Literature 4 proposes a titanium material for use in separators, the titanium material being formed by: subjecting a titanium alloy base material containing a platinum group metal to a pickling process by immersing it in a solution containing a non-oxidizing acid and an oxidizing acid, thereby causing concentration of the platinum group metal on the surface, and thereafter heat treating the titanium alloy base material in a low oxygen atmosphere. This results in formation of a mixture layer of the platinum group metal and a titanium oxide on the surface of the titanium material for separators, thereby providing the titanium material with good electrical conductivity, with the contact resistance being 10 mΩ·cm2 or less when an electric current of 7.4 mA is applied at a surface pressure of 5 kg/cm2.
In Patent Literature 4, reduction of contact resistance is accomplished by performing a heat treatment. This leads to thickening of the passivation film on the surface of the titanium plate, which results in problems of an increase in contact resistance and instability of contact resistance in the use for a long period of time. Furthermore, performing a heat treatment leads to increased costs, and what is more, it poses problems of reduced productivity and deformation by heat treatment due to the severe atmosphere conditions in the heat treatment. In addition, Non-Patent Literature 1 also discloses a titanium material of the type proposed in Patent Literature 4.
Patent Literature 5 proposes a titanium material for use in separators having a platinum group metal-concentrated layer on the surface thereof, the titanium material being formed by subjecting a titanium alloy base material containing a platinum group metal to a pickling process by immersing it in an acid solution containing a non-oxidizing acid.
Furthermore, in Patent Literatures 4 and 5, from a standpoint of inhibiting absorption of hydrogen into the titanium material, an acid solution containing an oxidizing acid is used in the pickling process. Because of this, in the titanium materials proposed in Patent Literatures 4 and 5, titanium oxides are formed in a layer under a redeposited platinum group metal layer, which poses a problem of a high initial contact resistance in the as-pickled state. Also, there are further problems in that, e.g., the thickening of the surface passivation film leads to an increase in contact resistance due to the influence of corrosion products or the like when the fuel cell is operated for a long time. In particular, in the invention disclosed in Patent Literature 4, the above-described problems become even more serious due to the heat treatment performed.